Vehicle cargo compartments for transporting bulk materials such as dirt, sand, grass clippings and the like are often covered with a canopy, often called a tarpaulin or tarp, during travel in order to avoid blowing out part of the load and thus creating a hazardous condition on the highway or contaminating the surroundings, or to prevent the load from getting wet. The deployed tarp needs to be held in place tightly to avoid being dislodged by the wind. Then the compartment needs to be uncovered to unload and load the bulk materials. The repeated and proper covering and uncovering of the compartment can be time-consuming.
Many different types of canopy deployment systems are available such as boom systems as disclosed in Petretti, U.S. Pat. No. 4,082,347, slider systems as disclosed in Merlot et al., U.S. Pat. No. 5,080,423, and side-rolling systems such as disclosed in Odegaard, U.S. Pat. No. 4,834,445 which deploys and retracts in a sideways manner from a double-wound laterally translating central roller. In the deployed configuration the roller is situated to span front-to-back over the middle of the open top of the compartment thus providing a middle support for the flexible canopy. The system in Odegaard uses an hydraulic motor to drive the deployment of the sheltering canopy off of the roller, and to drive retraction of the canopy back onto the roller. One advantage of hydraulic motors is that they are easily controlled in the forward and reverse direction by adjusting the pressure of the supplied fluid. However, hydraulic motors can have complex controls and be costly to manufacture, install and maintain. Further, leaking hydraulic fluid can contaminate other systems, cargo and the environment.
Some canopy systems use air driven motors. However such systems can be difficult to install and maintain due to lack of familiarity by operators. Further, some canopy systems are designed to tap into the existing vehicle air system operating the vehicle air brakes. Problems with the canopy air system can lead to more serious and potentially dangerous problems involving the vehicle brakes.
Some systems, such as the one disclosed in Eggers et al., U.S. Pat. No. 6,857,682, use a relatively inexpensive electric motor conveniently driven by the vehicle electrical system which typically operates using direct current (DC) and relatively low voltages of between about 12 VDC and about 24 VDC.
Often to reduce the cost of an electrical drive system, the type of electric motor selected will fall within a fairly narrow range of performance characteristics. This can be problematic in the cargo canopy field which is subject to changing conditions, harsh environments and rugged treatment. For example, the canopy mechanism can be impeded by obstacles protruding from the cargo or the accumulation of rust or debris throughout its various components. A frustrated operator may repeatedly activate the deployment and retraction motors in attempting to free the jammed mechanism or forcibly clear the obstacle. This puts great strain on the motors at the risk of burning their windings. Further, electrical systems operating in a hot, desert environment may tend to overheat more quickly then those operating in a cooler environment. In addition, a cargo canopy activating motor should be able to withstand a short burst of high current to deliver the torque necessary to overcome a temporary resistance without causing damage to the motor.
Thus, there can be great variation in the performance characteristics required by electric motors used by a system and their potential for early burnout may vary greatly. Although more rugged electric motors having wider performance characteristics are available, they tend to be more costly.
Exacerbating the problem, because of weight and cost concerns, electrical wire of no thicker than #6 gauge is often used in vehicle runs that can be in the tens of meters in length where DC electrical resistance becomes a large factor. Thus, higher DC currents are often used and, over time, there can be high power consumption, leading to the greater potential for motor burnout.
Installing a current sensitive circuit breaker or fuse between the source of electrical power and the motors has been the conventional way of protecting them. However, a circuit breaker responds mainly to the amplitude of the current drawn by the motors not necessarily to the amount of consumed power that might overheat the motor and damage the windings.
Fuses tend to respond somewhat better to the power consumed, namely blowing more quickly when the current is higher and more slowly when the current is lower. For example, a typical new 50 ampere fuse can blow within 10 seconds when the current passing through it is 160 amperes, but can take 40 seconds to blow when the current is 110 ampere.
Unfortunately, fuses can be more difficult, costly and time-consuming to replace. Furthermore, over time, a fuse will weaken so that eventually the fuse may blow when the power consumption is within acceptable limits. A frustrated operator may then decide to electrically bypass the fuse thus greatly increasing the risk of motor burnout and the potential for dangerous electrical shorts.
In some systems there is a dedicated motor for deployment actions and a separate motor to drive retraction actions. Using two or more motors thus makes the cost of a particular motor more important to the overall cost of the system.
Some canopy retracting mechanisms previously available are prone to erratic operation due to misalignment of canopy edges, uneven stretching of the canopy fabric, lack of synchronization between driving motors or the jamming of pulling and guiding components. Moreover, the canopy deploying and retracting motors need to be well balanced and relatively precise in order to assure a smooth operation of the mechanism under the rugged, all-weather conditions typical for cargo vehicles. For stability and exactness of alignment, prior mechanisms have been proposed which require complex boom and lever structures as disclosed in PCT International Publication WO 2005/005186 Hines.
In addition, as material is wound upon a roller or spool its effective diameter changes due to the material already wound on the spool. Thus, it can be difficult to predict the amount of material taken up for any given rotation. This can lead to uneven tensioning on the material which in turn can lead to misalignment.
Unfortunately because of the rugged environment and particular cost concerns of the cargo canopy field, those electrical and mechanical systems which work in a seemingly similar field such as retractable awnings may not provide any useful guidance for designs in the cargo canopy field.
The instant invention results from efforts to address one or more of the above problems with prior cargo canopy systems.